Process for recovering aerosol solids



Oct. 18, 1955 C, A. sToKl-:s

PROCESS FOR RECOVERING AEROSOL SOLIDS Filed April 14, 1951 IN V EN TOR.BYCZML wfm, @Wwf United States Patent Olice 2,720,939 Patented Oct. 18,1955 PROCESS FOR RECOVERING AEROSOL SOLIDS Charles A. Stokes, WellesleyHills, Mass., assignor to Godfrey L. Cabot, Inc., Boston, Mass., acorporation of Massachusetts Application April 14, 1951, Serial No.221,020

1 Claim. (Cl. 18S-121) This invention relates to the recovery of solidparticles from gaseous suspensions and comprises a new and improvedprocess for agglomerating such particles into aggregates of recoverablesize.

ln many industrial processes od gases contain varying amounts of veryline particles of solid materials requiring collection, either becausethey constitute the product of the process or because they arecontaminants when released into the atmosphere. When the particles areof colloidal size, i. e., less than about 0.5 micron in diameter, as isthe case with carbon black, ily ash and the like, collection presents aserious problem. Bag lters are eilective but are not entirelysatisfactory due to the relatively large pressure drop across the bags,to condensation of moisture around the bags, to frequent bag failure andto the necessity of shaking out the bags periodically. An alternativemethod of collection, electric or sonic agglomeration and cycloneseparation, is more commonly employed but does not achieve completecollection. ln fact, when the particles in the gas average less than 0.5micron in diameter and are present at dilutions of 5 grains per cubicfoot or less, a large traction will pass on through the agglomerator andcyclone separators and out the stack. And when the particles are ofmaterial that will not agglomerate at all in an electric or sonicagglomerator, the percentage of loss will be even higher.

The magnitude of the product loss is proportional to the neness andinversely proportional to the grain loading of the particles which arebeing produced, and in the case of ne furnace (FF) carbon blacks, forexample, the loss may amount to as much as 35%. Financially, this wasteof product is serious. Moreover, escaping product may create an annoyingsmoke nuisance to the surrounding countryside.

It is a dilcult problem to recover line smoke particles because thesettling rate of such particles diminishes rapidly as the particle sizedecreases. For example, a spherical particle having a diameter of l0microns and a density equal to two grams per cubic centimeter willsettle in still air at a rate of about 0.5 centimeter per second while aparticle of the same density having a diameter of l micron will settleat a rate of only .006 centimeter per second.

lt is the principal object of this invention to provide a novel andimproved process whereby substantially complete collection of thesmallest size gas-borne solid particles may be effected.

lt is also an object of this invention to provide a process foreffecting satisfactory agglomeration in a precipitator of substantiallyall of the solid particles suspended in gas streams passingtherethrough.

it is a further object of this invention to provide a process foreffecting agglomeration in an agglomerator of gas-borne solid particlesof materials which cannot otherwise be made to agglomerato byelectrostatic or sonic forces.

ln conventional practice gases containing iine solid particles areconducted through an aggiomerator, which may be of the well knownelectrical type, such as the Cottrell, or of the newer sonic type, suchas the ultrasonic, in which the particles are caused to agglomerato intolarger aggregates. As the orientation of the particles in the carriergases is far from uniform, aggregates or many sizes will be formed andsome of the particles fail altogether to agglomerato. ri`he largestaggregates will be sutciently heavy to fall out in the agglornerator butby far the largest proportion will pass on through. The gases, now withmost of the particles therein at least somewhat aggiomerated, are thenconducted through conventional cyclone separators in which most of theagglomerates will be caused to settle out. The remainder of theparticles are exhausted with the gases to the atmosphere.

ln accordance with this invention the eiiiciency of agglomeratie-n isvastly increased over that achieved by these procedures heretoforeknown. In fact, by the practice of this invention, all, or substantiallyall, or" the gasborne particles are caused to agglomerato intoaggregates so large that the major proportion of the particles will becollected in the agglomerator and any remaining proportion will becollected in the cyclones.

This invention is based upon the principle that when a liquid which willwet the surface of the particles, such as water, is introduced into thegas-solid aerosol stream in the form of a stable fog or mist, atremendous increase in agglomeration of the solid particles in theaerosol occurs. Whereas without the introduction of the mist largenumbers, and in the case of certain materials, all ot the solidparticles escape agglomeration entirely, with the addition or the inistvery few particles escape collision with one another and consequentagglomeration.

lt is, of course, old to separate solids from gases by means of liquidsprays or curtains which scrub the solids out of the gases. Thedisadvantage of such method is that large quantities of liquid arerequired from which the solids must be separated as a separate processstep. Additionally, extensive wetting of the solids may be harmful tothem. The process of this invention makes unnecessary the use of largequantities of liquid and does not cause wetting or' the productsul'icient to form a liquid slurry.

While best results are achieved when the liquid fog particles are in theaerosol range, a tine spray in which the droplets may be as large asabout Si) microns in diameter will serve to improve agglomerationelliciency. lt is merely necessary that the liquid droplets besurrciently small and suidciently disperse to remain gas borne, or toconstitute a stable fog, under the conditions of ow encountered in thesystem and not to knock out the solid particles in the manner of ascrubbing spray or curtain. in practice the tog will consist ofparticles non-uniform in size. The presence of a small number ofoversize droplets will not make the process of my invention inoperable.

The fog contemplated by this invention may be generated in anyconvenient manner. For example, where the gases carrying the solidparticles to be collected also contain substantial quantities of liquidvapors at elevated temperatures, the vapors may be reduced intemperature to their dew points and thereafter be uniformly cooled tocondense the moisture in accordance with well known principles. Or thefog can be separately generated and introduced into the gas stream. Orvery inely atomized liquid sprays in which the droplets are within thesize range discussed above can be introduced directly into the gasstream. However or wherever generated and introduced into the aerosolstream, the fog in substantial volume must be mingled with the gas-borneparticles in the agglomerator to accomplish the purposes of thisinvention.

ln the agglomeration chamber the intermingled constituents of the gasstream, namely, the carrier gases, solid particles and mist particles,are subjected to electrostatic,

sonic or other forces. These forces cause .the particles to collide withone another and agglomerate.

It is not fully understood -just why the addition of a fog or mistincreases the agglomerative eiect'. One reasonable explanation is thatthe incidence of collision is greatly increased, particularly in asuperimposed force iield, by

reason of the presence of an increased number of particles. Apparentlythe striking of Va solid particle by a liquid particle tends to preventthe escape of the solid and to increase theV chance lof its makingcontact with other solid particles. Additionally, the liquid probablyserves as Van adhesive so that when two solid particles with wetsurfaces come into contact with one another they adhere together. Yet athird phenomenon may be present. It

of opposite polarities in a system in which solid and liquid particlesarercontained favors the agglomeration of the solid particles.'

The novel process of this invention will better be understood andappreciated from theV following description of Y illustrative examplesthereof in connection with the accompanying -diagramatic drawings of'apparatus which Ymay advantageously be employed in carryingit outalthough the process is not restricted to this or to any speciic type ofapparatus.

ln the drawings, Figs. l and 2 are diagrammatic ow sheets. e

WhileV this description is presented with reference to the recovery ofcarbon black it will be understood that the process of this invention isnot'limited to the carbon black art the drawings, combustion, gases withentrained car- Y bon black are represented as beingconducted into acooling tower through a flue 12 leading from the carbon black producingfurnace 14. The tower 10 is provided with a-water spray forl cooling'theblack and gas to about ff450 Fahrenheit. vThe cooled gases with theirburden of carbon black'pass from the tower 10 through a ueV 16 into anelectrical or sonic agglomerator 18 and pair of cyclone separators 20and22. A mist or fog is introduced into agglomeratorlS from a foggenerator 24 or the fog Y mayV be'ntroduced at an intermediate rpoint asinto flue The fog generator maybe of any'conventional design whichvprovides a majority of liquid particles having a particle size-belowabout 80 microns in diameter. De-

pending upon the volume of combustion products con- Y tained infue 16,the volume of mist introduced will vary so Athat at all times the grainloading of the fog will be at least equivalent to theV grain loading ofthe carbon black y 5 is quite possible-that the distribution ofelectrical charges ing suicient density to be precipitated are removedbefore being atected by the mist particles. Or arsecond agglomerator canbe tacked onto the end of the conventional dry collection system and thefog can be introduced thereinto in a manner described above. Y

While, as has been said, .the process of this invention is suitable forthe collection of any iinely divided solid particles carried in any typeof gas it is particularly applicable to the collection of carbon blackwhich is one of Y the most diiiicult of solid substances to collect.Owing to Y the extreme subdivision of carbonrblack particles and thehighly dilute system in which they are contained, ag- ,Y Y glomerationis very difficult to achieve.

For instance, about the maximum grain loading of off-gases found whenproducing the coarsest 80-100 millimicrons average particle diameter) ofthe carbon blacks is 5v grains per cubic foot. This represents a yieldof about 12 pounds of carj-` bon black per 1000 cubic feet of naturalgas introduced into the producing furnace. As the particle sizedecreases so does the grain loading. For the smallest of the furnaceblack particles, below 40 millimicrons -in diameter, the yield of blackper 1000 cubic feet of natural gas-is only about 2 pounds and the grainloading is below 1 grain per cu. ft. Thus etlicient'agglomeration isinhibited by the twin factors vof srndl particle size and extreme dilu-Y It is thus obvious that the processofthis inven-V tion. tion becomesincreasingly important as the particle size of the carbon black producedbecomes smaller. Y ,p

The following example is illustrative of the practice of the process ofthis invention in the collection of 90-92Y scale (nigrometer) carbonblack having an average particle diameter inV the 80 millimicron range.

Gas Tun-e of EX' Percent Rate posure to Total Run No. Type ofAgglomeration Sonic or Elee- Cu. it. v trostatic `Collecper mm' force,Sees. I non 3 Blank 2 3 Fogonly 7 3 Sonic only 72 14 3 Sonic and fog 7270 3 Electrostatic only 44 70 4 Electrostatic and fog .Y 50 93V` Havingthus disclosed my inven'tion and described in,Y

' detail illustrativeexamples thereof, vI claim Yas new andY decontainedin ilue 16. Sonic or electrostatic force is inducedV in theagglomerato); causing the majority of the Y black particles toagglomerate and fallout of suspension. VThe gasesV then pass into thecyclone where additional Vcollection is accomplished.

Alternative apparatus for carrying out the-process of this invention andillustrated in Fig. 2 contemplates the substitution ofva coolingdevice-for the mist generator .24 of the system shown in Fig. l, wherebythe fog is obtained from the moisture content of the gases passingthrough ue pipe 16. In this embodiment the uid pipe16 is connected to acooler 17 which may contain brine-filled coils or carbon dioxide insolid or liquid state. Such a cooling arrangement will uniformly depressthe temperature'of the moisture-containing product gases. By uniformcooling, the moisture constituent of these gases is condensed into a fogor mist making possible the elimination kof the fog generator referredto above.

Further variations from the process above described are contemplated asbeing within the scope of this invention.

Additional cyclones may be interposed between cooling tower andagglomerator wherein the black particles hav- 4 sirek to secure byLetters Patent:

Aprocess for'recovering finely-dividedV solid Vproducts from gaseousfurnaceY effluents which comprises intermingling with said 'etiinents aliquid mist of` a grain loading of liquid droplets at least equal to.that Vof the solids therein but less than that sufficient to form aliquid slurry Vwith-the solid particles, owing said eiuents and mistVthrough an agglomerative force field and then throughra A cyclonicseparation zone and recovering the solid products in substantially drycondition from said separation zone by settling in an essentiallygaseous phase.

References Cited inthe tile of this patent UNITED STATES PriTnNr's'v

